(1) Technical Field
The present invention relates to radio-frequency micro-electromechanical switches (“MEMS”), and more particularly, to high-power radio-frequency MEMS signal contact switches.
(2) Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. Pin diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value less than 20 dB. This allows a signal to “bleed” across the switch even when the switch is open. Pin diodes and transistors have a limited frequency response and typically only respond to frequencies below about 20 GHz. In addition, the insertion losses and isolation values for these switches vary depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089, to Larson, disclosed a different class of microwave switch, termed the micro-electro-mechanical system (MEMS) switch. The MEMS switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These characteristics give the MEMS switch the potential to replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits.
The Larson MEMS switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature.
MEMS switches of the general type described above are, however, prone to premature failure. The cause of the premature failure is linked to the damage resulting from the impact of the armature contact with the substrate contact. Currently available MEMS switch designs have attempted to reduce the extent of damage. However, these designs still utilize beam type cantilever beam-type radio frequency (RF) MEMS switches which have double ohmic contact points that generally display a contact resistance of around 0.5 ohms. This contact resistance is the main limiting factor to the cycling number and power handling of existing MEMS switches.
More specifically, the dominant factor in limiting the lifetime of a switch is the edge contact of protrusion contacts upon activation. Edge contact allows less than 10% of the protrusion surface to contact with the bottom electrode. Thus, contact resistance is usually limited to around few hundred milliohms. Edge contact also causes excessive wear and tear during activation, resulting in an increased contact resistance, eventually causing catastrophic failure from heating.
Accordingly, there is a need in the art for a MEMS switch that is capable of high-power operation while avoiding premature failure due to increased impact per unit area. It is also desirable to have a MEMS switch which deters premature deterioration by reducing the amount of resistive heating due to increased current density through the small area of actual contact.